'Water freezes at 0 degrees C' is a universal truth, meaning that it is a statement accepted as a general truth.
In making this statement negative, we could say that:
Water doesn't freeze at 100 degrees C. Water freezes at 0 degrees C.
Or, the verb 'freezes' could be replaced by the negative form of its antonym which is 'boils'.
Water doesn't boil at 0 degrees C. Water boils at 100 degrees C.